Methods for producing diamond materials with enhanced heat conductivity

ABSTRACT

A method for growing a diamond film, substantially free of voids, having an average crystallite size greater than about 15 microns, a maximum intensity of the diamond Raman peak in counts/sec divided by the intensity of photoluminescence at 1270 cm -1  greater than about 3, a Raman sp 3  full width half maximum less than about 6 cm -1 , and a diamond-to-graphite Raman ratio greater than about 25, includes the steps of preparing a substrate by abrasion with diamond particles; placing the substrate in a CVD reactor; depositing diamond during a first deposition stage by providing an atmosphere consisting essentially of a mixture of about 200 sccm H 2  and 10 sccm CH 4 , at a pressure of about 90 Torr, providing between about 1,800 and 1,950 watts of microwave power at a frequency of about 2.45 GHz to ignite and sustain a plasma in the region of said substrate, and maintaining the substrate at a temperature of between about 625° C. and 675° C. for a period of time sufficient to form a diamond layer which is substantially continuous; depositing a diamond during a second deposition stage by providing an atmosphere consisting essentially of a mixture of about 200 sccm H 2 , 4.6 sccm CO, and 9 ccm of CH 4  at a pressure of about 90 Torr, providing between about 1,800 and 1,950 watts of microwave power at a frequency of between about 2.45 GHz and maintaining said substrate material at a temperature of between about 625° C. and 675° C. for a period of time sufficient to form a diamond layer having a desired thickness; and removing the substrate material from said CVD reactor.

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 08/095,314,filed Jul. 21, 1993, now U.S. Pat. No. 5,316,842, which is a division ofapplication Ser. No. 07/789,732, filed Nov. 8, 1991, now U.S. Pat. No.5,284,709, which is a continuation in part of application Ser. No.07/704,997, filed May 24, 1991, which is a continuation of applicationSer. No. 07/413,114, filed Sept. 27, 1989, now U.S. Pat. No. 5,075,095,which is a continuation of application Ser. No. 07/204,058, filed Jun.7, 1988, now U.S. Pat. No. 4,882,138, which is a continuation ofapplication Ser. No. 07/032,167, filed Mar. 30, 1987, now U.S. Pat. No.5,743,073.

FIELD OF THE INVENTION

The present invention relates to synthetic diamond materials andarticles. More specifically, the present invention relates to syntheticdiamond material and articles having thermal conductivity greater than17 watts/cm/K at about 20° C. Several preferred embodiments are taught,including materials with large grain size, modified isotopiccomposition, and combinations thereof.

THE PRIOR ART

It has long been known that diamond, in either its natural or syntheticsingle crystal form, is the best available conductor of heat at ordinarytemperatures. Specimens of natural type IIa diamond, selected for theirpurity and relative freedom from internal stress, have demonstratedthermal conductivity as high as approximately 20 watts/cm/° K. at roomtemperature. In comparison, the best metallic heat conductor (the bestknown heat conductor but for diamond) is silver, with a thermalconductivity of slightly in excess of 4.3W/cm/° K. The extremely highheat conductivity of diamond makes it an attractive candidate for use incooling high-power electronic devices.

The independent conduction mechanisms for heat and electric current indiamond underlie its utility as a thermal transfer element in manyelectronic applications. Thus for devices such as laser diodes, diamondcan provide required electrical isolation with simultaneous high thermalconductivity, enabling efficient removal of heat from sensitive devices.There is a strong need for high conductivity diamond to provide morecost-effective, efficient heat transfer components.

There is a wide range of potential thermal applications for diamond.However, the high cost and limited supply of suitable type IIa naturaldiamond crystals has restricted their use to applications whoserequirements can be met with very small crystals (i.e., having areas ofabout 1 mm²) and which can justify the high cost of selected naturalmaterial.

Synthetic diamond materials made by the classic high-pressure,high-temperature process are not suitable for many of the numerouspotential thermal applications which could be filled by diamond. Thehigh-pressure, high-temperature diamond materials cannot be made in thinfilm form, and are even more expensive in large sizes (i.e., greaterthan about 5 mm) than natural diamonds. Furthermore, single crystals asroutinely made for incorporating in abrasive cutting and grindingproducts are often "doped" with small amounts of incorporated nitrogenduring growth to improve their mechanical properties. These nitrogenimpurities reduce thermal conductivity by approximately a factor of two,depending on the details of synthesis.

High pressure, high temperature synthetic diamonds can be grown withoutnitrogen impurities, and therefore with high thermal conductivity, butproduction costs preclude economic fabrication of diamond thermaltransfer elements larger than several square millimeters in size.Therefore, it has not been possible to address applications for whichthe thermal properties of diamond are suited which require larger areas,or which would require diamond layers which must conform to irregularsurfaces.

Heat transfer in diamond occurs through propagation of phonons in thelattice. Phonons are essentially lattice vibrations which are quantizedin energy. This heat transfer mechanism is common to manycovalently-bonded dielectric materials (e.g., BeO, Al₂ O₃, etc.) anddiffers from that of metals, in which both heat and electric current aretransported by conduction electrons which are free to move throughoutthe metal. In contrast, covalently-bonded materials usually have veryfew free electrons at room temperature, and are therefore goodelectrical insulators.

The efficiency of heat transfer (phonon propagation) in diamond islimited by a number of independent effects. In general, any structuralor chemical inhomogeneity in the crystal lattice causes reflections ofphonons and consequent reduction in heat conductivity. In addition, aform of phonon interaction known as Umklapp scattering causes scatteringof colliding phonons even in perfect crystal lattices. This effect givesrise to a temperature-dependent reduction of thermal conductivity indiamond at elevated temperatures.

Lattice inhomogeneities known to cause phonon scattering and reducedthermal conduction in diamond include chemical impurities (incorporatednitrogen, boron, or other non-carbon elements), structuralnonuniformities (interstitial carbon atoms, carbon vacancies, and othertypes of defects such as stacking faults), boundaries within singlecrystals (twinning defects), boundaries between grains inpolycrystalline films, and isotopic inhomogeneities.

The isotopic scattering factor arises from the fact that natural carbonconsists of a mixture of isotopes, the predominant one being C¹²comprising about 99 atomic % in naturally occurring carbon forms. Theisotope C¹³ is next most abundant, with a concentration of about 1atomic %. Other isotopes (such as radioactive C¹⁴) are present invanishingly small concentrations and are not believed to play asignificant role in determining thermal conductivity of diamond.

The presence of 1% of C¹³ in diamond plays a significant part inlimiting its thermal conductivity because each C¹³ atom, which is about8% heavier than the predominant C¹² isotope, scatters phonons andimpedes their propagation through the lattice.

Experiments in measuring thermal conductivity of other covalently-bondedcrystals prepared to be isotopically pure have demonstrated that, underconditions where other limiting factors are insignificant, phononscattering due to isotopic effects can strongly limit heat conduction.These effects have been noted in diverse materials such as germanium andlithium fluoride. It has been long predicted that isotopically purediamonds might show enhanced thermal conductivity, and recentexperimental results have shown that single diamond crystals withreduced C¹³ content show thermal conductivity increased by up to 50%over the best mixed isotope crystals.

Measurements of polycrystalline diamond film thermal conductivity havebeen published and, with a single exception, have not to date shown thelarge thermal conductivity noted in the best natural diamond specimens(Type IIa crystals). It has been speculated that small grain size and/orlattice defects have limited diamond film thermal conductivity, butmeasurements of thermal conductivity of diamond films, synthesized underconditions expected to maximize thermal conductivity, with apparentgrain sizes exceeding 25 μm disclosed maximum thermal conductivitylevels no greater than those measured for films having grain sizesmaller by a factor of ten. Lattice defects of an unidentified natureare speculated to be the limiting factor in CVD diamond thermalconductivity, and are specifically suggested as a factor masking theenhanced thermal conductivity expected of isotopically enriched films.The single exception appears in T. Morelli, Phonon-defect scattering inhigh thermal conductivity diamond films, Applied Physics Letters, Vol.59, No. 17, Oct. 21, 1991, pp. 2112-2114, but this reference provides nodisclosure regarding how to make the films reported therein.

In summary, despite the general knowledge in the art concerning themechanisms presently understood to affect thermal conductivity ofdiamond, no published data by expert practitioners in the field of CVDdiamond growth have yet shown how to synthesize diamond films withthermal properties approaching those known to be available in the bestsingle-crystal natural material.

BRIEF DESCRIPTION OF THE INVENTION.

According to the present invention, synthetic diamond material andarticles are taught having thermal conductivity greater than 17watts/cm/K measured at about 20° C. Several preferred embodiments aretaught, including materials with a grain size larger than about 15microns, modified isotopic composition, and combinations thereof.Materials comprising composite compositions of diamond and non-diamondmaterials, and articles made therefrom according to the presentinvention, also exhibit high thermal conductivities.

These materials make possible the transfer of heat with unprecedentedefficiency in a wide variety of applications. Our invention is thesynthesis of diamond materials, which if manufactured to manifest aparticular set of characteristics, exhibit thermal conductivitysubstantially greater than those manufactured by other practitioners ofthe art, and which in fact exceed thermal conductivity values seen inthe best natural specimens.

The materials and articles according to the present invention are fullydense films, free of voids, and characterized by an intensity ratio ofdiamond- Raman-peak-to-photoluminescence background intensity greaterthan about 20, intensity patio ofdiamond-Raman-peak-to-Raman-graphite-peak greater than about 25,full-width at 1/2 maximum intensity of diamond Raman peak less thanabout 6 cm⁻¹, average crystallite diameter greater than about 15microns. Further the films and articles according to one aspect of thepresent invention preferably have C¹³ isotopic concentration less than0.05 atomic %.

Diamond and composite materials prepared to exhibit the abovecharacteristics will exhibit thermal conductivity in excess of 17 W/cm/°K. measured at about 20° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the principal Raman features of carbon.

FIG. 2 is a Raman spectrum of a high-quality diamond film with little orno detectable graphitic impurities.

FIG. 3a is a graph showing thermal conductivity in diamond as a functionof diamond Raman-peak-to-photoluminescence background intensity ratio.

FIG. 3b is a graph showing thermal conductivity in diamond as a functionof intensity ratio of diamond-Raman-peak-to-Raman-graphite-peak.

FIG. 3c is a graph showing thermal conductivity in diamond as a functionof full-width at 1/2 maximum intensity of diamond Raman peak.

FIG. 4 is a graph showing the correlation between diamond grain size andthermal conductivity.

FIG. 5 is a graph showing thermal conductivity of diamond film as afunction of film thickness and provides a comparison between the thermalconductivity of diamond films and single crystal (IIa) diamond.

FIG. 6 is a diagram schematically showing gas delivery according to apresently preferred embodiment of the invention, showing the position ofthe showerhead relative to the plasma.

FIG. 7 is a schematic representation of a cross sectional view of amixed fiber and particle composition consolidated with high thermalconductivity polycrystalline diamond.

FIGS. 8A, 8b, 8c, 8d, and 8e illustrate a process for forming compositearticles having sectional portions of arbitrary thickness by addition ofnew particulate material to the surface of the article undergoingconsolidation.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Those of ordinary skill in the art will realize that the followingdescription of the present invention is illustrative only and not in anyway limiting. Other embodiments of the invention will readily suggestthemselves to such skilled persons. As used throughout thisspecification and in the claims herein, the term "chemical vapordeposition" shall be referred to by the known abbreviation "CVD".

According to the present invention, polycrystalline diamond materialsand processes for making the films are disclosed. Compositediamond/non-diamond films are also disclosed herein. The diamond andcomposite materials produced by the present invention have thermalconductivity greater than 17 watts/cm/K measured at about 20° C.

One of the underlying necessary conditions for high-thermal-conductivitydiamond is a high degree of structural perfection, as disclosed by Ramanspectroscopy. It has been discovered that there is a correspondencebetween photoluminescence and Raman spectroscopy data and structuralcharacteristics of diamond materials. According to the presentinvention, the required structural perfection of diamond films may beidentified by specifying quantitative limits on certain Raman andphotoluminescence features.

A common means of nondestructive analysis of diamond films is the use ofRaman spectroscopy. This is an optical technique in which a fixed energy(wavelength) of illumination is directed onto a sample to be analyzed.Raman scattering in the sample volume produces light with energydifferent from the excitation energy by either the sum or the differenceof phonon energy levels in the solid being probed and the excitationenergy. One- and two- phonon Raman scattering effects are known, inwhich the Raman signals are displaced from the excitation energy by ±1or ±2 phonon energy levels.

Phonon energy levels in covalent solids like diamond are sensitive tothe details of chemical bonding. Thus, diamond-bonded carbon produces aRaman shift (energy displacement) of about 1332.5 cm⁻¹ (a unit ofenergy), while graphitically-bonded carbon produces a shift of about1550 cm⁻¹. The shape, position, and intensity of Raman spectral peaksprovide information about sample stress, temperature, crystallite size,and defect density.

FIG. 1 shows a schematic view of the principal Raman features of carbon,and FIG. 2 shows a Raman spectrum of a high-quality diamond film.Practitioners skilled in the art will recognize that the Raman spectrumshown in FIG. 2 indicates a diamond film with little or no detectablegraphitic impurities. However, there is an appreciable "background", orbroad-based signal underlying the Raman diamond peak which indicatesthat the film has a significant degree of disorder at the atomic level.This background signal is generated by photoluminescence mechanisms atdefect sites rather than Raman scattering from carbon atoms. Thesedefects can be optically active and serve as a crude measure of disorderin the crystal.

The inventors have discovered a correspondence between Ramancharacteristics and thermal conductivity of diamond films. It has beendiscovered that diamond films having optimal thermal properties may becharacterized by an intensity ratio of diamondRaman-peak-to-photoluminescence background intensity greater than about20, intensity ratio of diamond-Raman-peak-to-Raman-graphite-peak greaterthan about 25, full-width at 1/2 maximum intensity of diamond Raman peakless than about 6 cm⁻¹, and a figure of merit (consisting of the maximumintensity of the diamond Raman peak in counts/sec divided by theintensity of photoluminescence at 1270 cm⁻¹)>3.

Together, FIGS. 3a-3c illustrate the maximization of thermalconductivity in diamond according to the present invention. FIG. 3a is agraph showing thermal conductivity in diamond as a function of diamondRaman-peak-to-photoluminescence background intensity ratio. FIG. 3b is agraph showing thermal conductivity in diamond as a function of intensityratio of diamond-Raman-peak-to-Raman-graphite-peak. FIG. 3c is a graphshowing thermal conductivity in diamond as a function of full-width at1/2 maximum intensity of diamond Raman peak. Together, FIGS. 3a-3cillustrate the maximization of thermal conductivity in diamond accordingto the present invention.

The crystallite size in diamond films increases as the films growthicker, given constant deposition conditions. Diamond films of varyingthicknesses have been prepared and their crystallite sizes and theirthermal conductivity values have been measured. According to the presentinvention, a strong correlation exists between crystallite size andthermal conductivity. The diamond materials fabricated according to thepresent invention should have average grain size larger than about 15microns. FIG. 4 is a graph showing the correlation between diamond grainsize and thermal conductivity.

In addition, the observed correlation between film thickness and grainsize, coupled with the above correlation of thermal conductivity withgrain size, may be used to determine the maximum thermal conductivity ofdiamond being produced in these experiments. This parameter is termedthe "local thermal conductivity" to distinguish it from the composite,or mixed thermal conductivity with grain size. The conductivity of thelargest grain CVD diamond films exceeds that of the best single-crystalnatural material. FIG. 5 is a graph showing thermal conductivity, ofdiamond film as a function of film thickness. The horizontal line onFIG. 5 provides a comparison between the thermal conductivity of diamondfilms and single crystal (IIa) diamond.

Finally, when diamond films are synthesized under conditions which giverise to the measured Raman values cited above, and in addition thecarbon precursor materials used are isotopically purified such that theresulting film has substantially less than 0.1 atomic % C¹³ (i.e.,substantially more than 99.9 atomic % C¹²), even greater thermalconductivity results as isotopic scattering effects are reduced.

As will be apparent to those of ordinary skill in the art,diamond/non-diamond composite materials, such as those described inco-pending application Ser. No. 07/704,997, filed May 24, 1991, willbenefit from incorporation of the material disclosed herein. Inparticular, synthesis of such composite materials, which may incorporatenon-diamond material as well as diamond material, can be carried outover a wider range of non-diamond/diamond material ratios, enhancing therange of properties available from such composite materials. Thus, ifthe ratio of nondiamond/diamond material in a desired composite is fixedby the need to achieve a certain thermal expansion coefficient, it willbe possible, using the high thermal conductivity diamond material of thepresent invention, to achieve a higher thermal conductivity in thecomposite while maintaining the required thermal expansion coefficient.

The high thermal conductivity diamond and diamond-infiltrateddiamond/non-diamond composite materials of the present invention may befabricated using CVD techniques. A CVD reactor in which these materialsmay be produced is of a type whose design has been optimized for theproduction of high-quality diamond films. In particular, the reactor isconfigured to assure control over gas dynamics such that all, orsubstantially all, of the gas mixture injected into the reactor isinjected directly into the plasma ball, such as by means of a"showerhead" gas distribution structure of a type similar to many knownto those who are skilled in the art of chemical vapor deposition. Theshowerhead is preferably positioned over the plasma zone such that thegas flows from the showerhead apertures directly through the plasma zoneand impinges immediately on the deposition surface. FIG. 6 is a diagramschematically showing gas delivery according to a presently preferredembodiment of the invention, showing the position of the showerheadrelative to the plasma.

Reactor parts should be fabricated from materials chosen to excludecarbon, to eliminate a potential uncontrollable source of carbon duringformation of the diamond material produced. Materials suitable for useinclude molybdenum, quartz, and stainless steel. It is preferable thatreactant gas purity levels be controlled such that no single gasexhibits residual impurity levels greater than 1/10,000 parts by volume.Successful production of high thermal conductivity diamond material hasbeen achieved using CVD apparatus as described herein.

EXAMPLE 1

In a plasma CVD reactor suitable for deposition of high-quality diamondfilms, a diamond film was deposited on a single-crystal silicon waferusing a two-stage deposition protocol. The single-crystal siliconsubstrate was first prepared for deposition by scratching using diamondpowder in a manner well-known to those skilled in the art.

In the first stage of the deposition, which was run for a duration of 30minutes, a mixture of 200 sccm H₂ and 10 sccm CH₄, at a pressure of 90Torr, was allowed to flow through the deposition apparatus. Betweenabout 1,800 and 1,950 watts of microwave power at 2.45 GHz were appliedto the deposition chamber, causing the ignition of a sustained plasma.The substrate was held to a temperature ranging between about 625° C.and 675° C.

During the second stage of deposition, the gas mixture was altered toinclude 200 sccm H₂, 4.6 sccm CO, and 9 sccm of CH₄. Deposition wascarried out for a duration of 451 hours. This protocol resulted indeposition of a diamond film with a thickness of approximately 350microns.

The diamond film was released from the silicon substrate usingwell-known chemical etch methods. Using a Nd:YAG laser, a sample was cutfrom the film and polished using conventional diamond polishing methods.Thermal conductivity was measured and was found to be 17.2 W/cm/° K. ata measurement temperature of approximately 20° C. Peak thermalconductivity of the material was determined to be 23 W/cm/° K., which isin excess of reported single-crystal heteroisotopic diamond (i.e., 20W/cm/° K.), and far in excess of other reported CVD diamond films.

Those of ordinary skill in the art will appreciate that the preferredembodiment described above, comprising a two-stage deposition process,is not a limitation in the manner in which a diamond material having theattributes described herein may be formed. Another embodiment comprisesseeding a substrate, such as the silicon wafer described above, withdiamond particles at a density exceeding 10⁷ particles/cm², and thensubsequently exposing the seeded substrate to the deposition processrecited as stage two in the two-stage process described herein. Adiamond material is produced which is substantially free from voids andwhich meets all other criteria set out herein for high thermalconductivity diamond material.

It is believed that this process would result in a diamond materialhaving an even higher thermal conductivity if performed using anisotopically pure (i.e., about 99.95 atomic % C¹²) carbon source.

Diamond/non-diamond composite materials may be produced according to thepresent invention. As disclosed in co-pending application Ser. No.07/704,997, filed May 24, 1991, such materials may be formed by CVDinfusion of diamond into the voids of preforms comprisingdiamond/non-diamond particulate mixtures.

As shown in FIG. 7, the particles may be either solid particles 12 orcomposite particles 18, i.e., particles 18 formed from a first materialcoated with a layer 20 of a second material. The particles 21 (or thesurface portion 20 of composite particles 13) may comprise any materialwhich is compatible with polycrystalline diamond deposition techniques.It is presently contemplated that particles comprised of materials whichwork well for diamond deposition as substrates in regular flat surfaceform, including, but not limited to, diamond, silicon nitride, tungsten,tungsten carbide, molybdenum, and silicon will provide suitable particlesubstrates for consolidation into the compositions of the presentinvention. It is believed that particulate quartz and alumina may alsobe consolidated according to the present invention. Particle mixtures ofdifferent materials are also consolidated within the scope of thepresent invention.

According to one embodiment of the present invention, enhancement ofthermal conductivity and strength of polycrystalline diamond compact,such as that sold under the trade name Compax by GE Superabrasives ofWorthington, Oh., is accomplished by further consolidating it using thepresent invention. This starting compact material is created by firstmetal-coating diamond particles and then pressing them together underhigh pressure and temperature. The metal coating melts and allows theunderlying diamond grains to grow partially together. After cooling, themetal matrix may be leached away with a solvent, such as an acid ormixture of acids, leaving a porous network of diamond particles.Consolidation of this material according to the present inventionfurther enhances the physical properties of this material.

The consolidation process of the present invention may be used toconsolidate a broad range of particle sizes consistent with the need toemploy infiltrated diamond material having a grain size larger thanabout 15 microns. Those of ordinary skill in the art will appreciatethat a lower bound exists for particle size at the point where theamount of inter-particle space available for infiltration with diamondhaving grain sizes within the bounds of the present invention will betoo small to allow the formation of diamond coatings with grain sizesgreater than about 15 microns. It will be further appreciated that theparticle size at which this occurs is not solely a function of particlesize, but depends also on particle shape and the degree to whichparticles are packed tightly together, both of which can be highlyvariable. To the extent that this situation pertains, the diamond matrixmaterial produced by CVD infiltration of such a particulate preform willexhibit thermal conductivity which is gradually reduced in accordancewith the reduction in grain size of the starting particles. Sizing ofthe starting particles thus provides another vehicle for controlling thethermal conductivity in composite articles according to the presentinvention.

It is presently believed that the optimum particle and pore size willdepend on the application to which the finished product will be put. Forexample, the particle and pore size distribution which is best formaximum consolidation will probably be less useful for producing aporous material for use as a filter. Generally, if the particles are toosmall, the surface will grow over with a solid diamond layer andterminate growth in the interior. If the particles are too large,impractically long deposition times may be required to achieve goodconsolidation because pore sizes will be large. This consideration willalso affect the size of the objects to be fabricated according to thepresent invention. The center of thicker objects must be assured ofconsolidation prior to the completion of surface consolidation whichcuts off the flow of reactant gases.

Composite materials fabricated by infiltration of high-thermalconductivity diamond formed according to the present invention may beformed with a desired degree of porosity limited on the low end bystructural integrity considerations and on the upper end by desiredthickness. The infiltrated diamond component of these compositematerials will exhibit substantially no voids.

Diamond particles of 10μ and 100μ and silicon carbide particles of 100μin diameter have been successfully consolidated at an average depositionrate of approximately 1μ/hour. At this deposition rate, the 100μparticles form better compositions than the 10μ particles becausedeposition at and immediately adjacent to the surface of the 10μparticles causes premature closure of the surface porosity (i.e., withinabout 6-10 hours), resulting in cessation of deposition, and thereforeof consolidation, within the interior of the material.

It is believed that irregularly shaped (i.e., particles with fracturesurfaces and aspect ratios of up to about 3 to 1) particles of naturaldiamond dust and ordinary synthetic silicon carbide abrasive particleswill work satisfactorily. It is presently believed that a uniform poresize leads to the greatest densification under forced flow conditions.

Particle shape will also affect the properties of the finishedcomposite. If particles have a large length/diameter ratio (in excess ofabout 7/1), they will behave more like fibers and may improve thefracture toughness of the composite material. Other properties, likescattering of polarized light, and directionality of thermalconductivity, may also be affected by use of fiber-like particles.

The present invention also contemplates consolidation of mixtures ofdiamond and non-diamond particles. Varying the proportions of diamondand non-diamond particles in compositions made according to the presentinvention allows for the control of important physical properties of theresulting material. For example, thermal expansion, and thermal andelectrical conductivity of compositions made according to the presentinvention may be controlled by altering the ratio of diamond tonon-diamond particles from which the composition is made. At a givenporosity, the thermal impedance of the composition will be approximatelythe weighted average of the thermal impedances of the componentmaterials, weighted by volume percent of composition. The high thermalconductivity diamond material of the present invention will furtherenhance thermal conductivity of the composite materials.

In some circumstances, it may be desirable or necessary to employparticles or fibers which are themselves composites of two or morematerials. For example, nickel and iron are poisonous to the diamonddeposition process. Consequently, if it is desired to consolidate nickelor iron particles with diamond material, it may be necessary to coateach particle prior to final diamond consolidation with a material whichpresents a hospitable surface for diamond deposition. Thus a thin layerof metal such as molybdenum or ceramic such as silicon carbide, both ofwhich are known to support diamond deposition, may be applied to theiron or nickel particles to prepare them for consolidation with diamondas earlier described. Additional areas of utility for compositeparticles as precursors to densify with diamond include modification ofelectrical, thermal, or mechanical properties through use of appropriatecoatings.

A similar process may be used to form composite fibers which may then beused for consolidation with diamond. In this instance, use of compositefibers not only allows use of inhospitable fiber materials and/ormodification of selected properties, but also makes available compositefibers which are substantially all diamond for use as elements in themanufacture of a diamond-fiber-reinforced, diamond-consolidatedcomposite material.

For example, when 5μ fibers of silicon nitride (HPZ silicon nitride,available from Dow Corning) are coated with approximately 25μ of diamondthrough chemical vapor deposition means, a 50μ diameter fiber is formedwhose greater portion consists of diamond, and whose properties aresubstantially those of a pure diamond fiber. This is useful becausecurrent technology does not permit the economic manufacture of purediamond fibers.

Because a critical factor in determining the mechanical properties offiber reinforced materials is the behavior of the interface between thefiber and the surrounding matrix, it may be desirable to modify thesurface chemistry of a diamond composite fiber by applying an outerovercoat layer of an appropriate material. For example, use of a siliconcarbide overcoat or a thin metal layer such as molybdenum will increasethe adhesive strength between the diamond composite fiber and thesurrounding matrix.

FIG. 7 is a schematic representation of a cross sectional view of amixed fiber and particle composition 22 consolidated withpolycrystalline diamond 14. Non-diamond fibers, for example siliconcarbide, silicon nitride, or alumina, are shown at reference numeral 24.Diamond-consolidated fibers are shown at reference numeral 26,comprising a non-diamond fiber 24 coated with a layer of diamond 28.Diamond-consolidated fibers comprising a non-diamond fiber 24, coatedwith a layer of diamond 28 and overcoated with non-diamond layer 32,comprising substances, for example quartz, silicon carbide, siliconnitride, or alumina, are shown at reference numeral 30. For illustrativepurposes only, the mixed fiber and particle composition 22 of FIG. 7 isshown comprising several types of fibers and particles which may beconsolidated according to the present invention. Those of ordinary skillin the art will recognize that an actual mixed fiber and particlecomposition 22 formed according to the present invention may contain oneor more of the types of particles and/or fibers actually shown in FIG.7.

There appears to be no inherent limitation regarding the ratio ofdiamond to non-diamond particles which may be consolidated according tothe present invention, so long as the material of which the non-diamondparticles are comprised is compatible with diamond 14 deposition, and isof a size range which will allow proper consolidation. With respect tosize range ratios, there will be various optimum particle sizedistributions depending on specific process operating conditions. Byproperly tailoring the pore size distribution as a function of positionwithin the mass being consolidated to compensate for deposition ratedifferences, it may be possible to achieve higher densification thanwith a simple uniform pore size distribution.

For example, a higher overall degree of consolidation may be achieved byfabricating preforms such that pores most distant from the source ofreactant gases are smaller than those closest to the source of reactantgases. This average reduction of pore size with increasing distance fromthe reactant gas source compensates for the reduction in growth ratewhich occurs with increasing distance from reactants. This technique maybe used in combination with imposed thermal gradients and/or controlledgas flow methods, but is particularly useful when thermal gradientsand/or controlled gas flow techniques cannot be employed due to specificapplication or engineering requirements.

One major controlling factor in the process according to the presentinvention is the deposition rate. For example, if the average particleand pore size are about 100μ, a growth rate of 1 μ/hr will close off theaverage pore in 50 hours. A presently preferred maximum growth rate isabout 1% of the average particle size, expressed in microns/hr. Thus, aparticle preform having average particle sizes of about 100μ can beconsolidated using a process with a deposition rate of about 1μ/hr. Thisis a rule of thumb rather than a hard and fast rule, and departures fromthis rule will be fairly common, depending on particle shape, whethermore than one particle size is present, and on whether thermal gradientsand gas flows in the process are arranged to modify local depositionrates.

For example, a way to increase the degree of consolidation of acomposition according to the present invention is to arrange for theregion most distant from the plasma or other reactant source to be thehottest. Because deposition rate is a strong function of temperature,this compensates for the tendency for the regions nearest to the plasmato grow more quickly, and postpones premature termination ofconsolidation resulting from closure of gas diffusion passages. Underthese circumstances, a faster deposition rate may be useable.

Although the foregoing discussion of consolidation of particulates bychemical vapor infiltration has focussed on the use of polycrystallinediamond as a matrix material, those of ordinary skill in the art willrecognize that operative embodiments of the present invention can beused to consolidate diamond particles in the variety of forms discussedherein by chemical vapor infiltration of non-diamond matrix materialssuch as silicon carbide. This process produces a further variety ofdiamond composite materials having desirable properties and broadens thecommercial utility and application of diamond composite materials. Aspecific example of such a system is the consolidation of diamondparticles by chemical vapor infiltration of silicon carbide matrixmaterial, using methane and silane gas chemistry as is well known in theart.

The particles may be precleaned by rinsing them with isopropyl alcoholand drying them on filter paper. The particles may then be premixed asrequired. To prepare particle mixtures, the appropriate amounts areweighed out in the dry state and the weighted amounts are transferredinto a beaker or crucible for subsequent mixing.

Small amounts of a liquid, such as isopropyl alcohol or polyvinylalcohol are preferably added to the dry particle mix to form a pourableslurry. The properties of the ideal slurry-forming liquid, or vehicle,include somewhat elevated viscosity (to prevent rapid settling ofparticles after mixing) and complete, residue-free evaporation from theslurry after pouring into a mold.

The slurry is then poured into a mold having the desired shape of thefinished consolidated article and the slurry vehicle is allowed toevaporate, either unaided, or with the assistance of vacuum and/or heat,to leave a particle preform. Too rapid vehicle removal causes bubble orvoid formation in the finished preform. For uniform thickness, the moldmust be kept level. This is especially important for thin, wide items,as slight tilts cause the slurry to pile up at one side of the mold.

On the other hand, deliberate mold tilting may be employed to obtainlinear thickness variation if desired. In addition, a circular mold maybe spun to obtain a parabolic thickness distribution through theinteraction of centrifugal force and gravity.

Mold material and surface finish can be important, depending on thedesired result. Ideally, a mold should be made of a material to whichdiamond does not strongly adhere or grow upon, to ease post-depositionmold separation. The mold surface texture is replicated in the adjacentconsolidated material, so smooth finishes may be obtained in thecompleted material if a mold with a polished surface is employed. Moldscan also include shapes of various types which give contour and reliefto the finished material. This is an especially important capability inthat it reduces or eliminates the need for post-deposition machining, animportant cost-reduction consideration in view of the hardness of thefinished product.

A circular copper gasket with an inner diameter of 2.25 inches centeredon the polished surface of a silicon wafer has been shown to functionsatisfactorily as a mold. A copper gasket about 2 mm thick has beenemployed, although other thicknesses may be employed. The gasket issimply placed on the wafer and is kept in place by gravity. The siliconsurface is extremely flat and smooth, and is compatible with the diamonddeposition environment. It is easily etched away following deposition.

The isopropyl alcohol vehicle may be removed by evaporation acceleratedwith gentle heating. The mold is placed on a levelled hot plate, and theslurry is poured into the mold. Generally, the seal between the coppergasket and the wafer surfaces is good enough that very little liquidleaks out. The particles are too large to be carried along small leakpaths. After the slurry is poured and levelled (if needed), the hotplate is turned on and the slurry temperature is allowed to increase toabout 40-45° C. Evaporation takes about 2 hours.

When the vehicle has evaporated, the copper gasket is carefully removedby lifting it vertically off the wafer. This leaves a disc ofloosely-bound particles on the wafer. The copper gasket is removedbecause copper is not compatible with the diamond deposition process.The preform particle composite disc is placed, still on the underlyingwafer, in a diamond deposition system as described above herein.

The power required to maintain a specific temperature changes duringconsolidation as the thermal conductivity and radiation properties ofthe consolidating material change during the process.

After initial consolidation of the particulate preform article has beenachieved and the preform article mass has acquired an enhanced degree ofmechanical integrity (about twelve hours), the second stage ofdeposition preferably includes rotating the support platform upon whichthe preform article rests. In a presently preferred embodiment, therotational speed is between about 60 and 2,000 rpm, preferably about300-600 rpm. In a .presently preferred embodiment, the plasma may besimultaneously displaced from the center of the support platform to aposition at about one half the radius of rotation of the preformarticle. The combination of rotation and plasma displacement providesmore uniform heating of the preform article and leads to betterdeposition uniformity. In another preferred embodiment, the plasmaposition may be rapidly varied over the rotating or stationary preformthrough means of phase modulation of either or both the incident orreflected microwave energy which define the plasma location within thedeposition chamber. This has the effect of increasing deposition areaand uniformity.

After continuation of the second deposition phase for betweenapproximately 48 to 168 hours, depending on the degree of porositydesired, the preform thickness, particle size, and growth rateachievable under required process conditions, during which microwavepower is increased to maintain sample temperatures, deposition isterminated by switching off the microwave power supply and discontinuingthe flow of methane gas. The excess methane gas may be removed from thechamber by momentarily opening a high flow rate valve between thechamber and the vacuum pump. The sample is allowed to cool, preferablyunder flowing hydrogen gas at a pressure of about 100 torr.

After cooling and removal from the reaction chamber, the silicon orother substrate support may be removed by etching. Where the supportsubstrate is a silicon substrate, it may be dissolved in a 2:1 mixtureby volume of concentrated reagent grade HNO₃ and HF, which is sufficientto remove the silicon substrate without attacking the densified ceramicdiamond article.

The composite articles of the present invention may be formed in holesor other recesses of non-diamond materials prepared by etching, drillingor other mechanical processes to accept regions of diamond ornon-diamond solid or composite particles. According to this aspect ofthe invention, a substrate material, such as silicon carbide, beryllium,aluminum, or other material compatible with diamond forming processes isprepared by forming one or more holes or other recesses. Particles to beconsolidated are placed in the hole or recess and the loaded substratematerial is placed in the diamond growth reactor. Consolidation of theparticles is performed as disclosed herein. Where the substrate materialis one to which deposited diamond will adhere, no other means need beused to secure the position of the consolidated mass in the hole orrecess of the substrate material. Where the substrate material is one towhich diamond has poor adhesion, the hole or recess can be formed usingundercuts, i.e., its diameter increases with its depth below itssurface. The diamond consolidated particle mass will be interlocked withthe substrate material and thus be held thereto by a mechanically stablebond.

One limitation on the production of diamond consolidated particlecompositions by chemical vapor infiltration is that there exists a limiton the thickness of the article which can be produced. This limitationoccurs because, at some thickness determined by the particular processparameters in use, it is no longer possible to transport the necessarygrowth species from the surface of the article to its interior. Thisplaces limits on the commercial usefulness of diamond compositematerials because there are items which, by their nature, requiresubstantial section thicknesses for their manufacture. Examples of sucharticles which may require thick sections include turbine blades,metrology gauge blocks, and ceramic vacuum tube envelopes.

According to another aspect of the present invention, articles havingsectional portions of arbitrary thickness may be formed if the processis carried out by addition of new particulate material to the surface ofthe article undergoing consolidation. Such a process is illustrated inFIGS. 8a-8e, a series of cross sectional views of a diamond consolidatedarticle at various points in the process.

Referring first to FIG. 8a, the initially unconsolidated particles areshown formed into a preform of a desired shape. FIG. 8b shows thearticle after particle consolidation has been completed according to theteachings of the present invention.

Next, as shown in FIG. 8c, additional particles are added to the surfaceof the now consolidated article and further particle consolidation ofthe newly placed particles is accomplished, resulting in the thickerconsolidated article shown in FIG. 8d. It is presently contemplated thattwo possible modes of operation of this phase of the process arepossible. In a first batch mode, the consolidated article of FIG. 8b isremoved from the deposition chamber, or the chamber is at least opened,and the additional particles are placed on the surface of the article.The deposition chamber is then closed and further deposition isperformed to consolidate the new particles. Performance of thisvariation in the process will result in discernable boundaries withinthe article at the locations across the cross section of the articlerepresenting the surfaces where the new particles were placed.

In certain applications, the boundaries within the article may havedeleterious effects on the strength of the finished composite article.Where these features are undesirable in the finished article, acontinuous process may be performed wherein additional particles arecontinuously added to the surface of the article at a rate determined bythe progress of the consolidation process. For example, a consolidationprocess may be carried out in which particles to be consolidated are,dispensed in a slow continuous fashion using any of a number of particledispenser mechanisms, including vibrating hoppers, impact hoppers,screw-driven particle feeds, or gas puff particle feeds. The particlefeed rate is adjusted such that the average particle feed rate is notgreater than the effective consolidation rate, i.e., such that particlesare substantially fully incorporated into the underlying consolidatedmass before an excessive depth of new particles are added. In thisfashion, a continuous densified layer is formed by accretion ofparticles and infiltration of polycrystalline diamond matrix material.The particulars of particle feed rate are determined by such factors asmatrix deposition rate, particle shape, particle size, and reactiontemperature. A further advantage of continuous particle feed andconsolidation is that the average depth of porous material which must beconsolidated can be kept relatively shallow compared with the sectionsconsolidated in batch processes, and therefore will be less subject tomatrix material compositional variation with depth, producing a moreuniform product.

Where batch processing is employed, the process steps illustrated byFIGS. 8c and 8d may be repeated as many times as necessary to obtain afinished article having a desired thickness as shown in FIG. 8e. Wherecontinuous processing is utilized, the particle feed is stopped whenenough material is present in the article to achieve the desired endthickness.

While embodiments and applications of this invention have been shown anddescribed, it would be apparent to those skilled in the art that manymore modifications than mentioned above are possible without departingfrom the inventive concepts herein. The invention, therefore, is not tobe restricted except in the spirit of the appended claims.

What is claimed is:
 1. A method for forming a coating on a substrate toform a coated article, said coating consisting essentially of diamond,said diamond characterized by being substantially free of voids, havingan average crystallite size greater than about 15 microns, said coatinghaving a maximum intensity of the diamond Raman peak in counts/secdivided by the intensity of photoluminescence at 1270 cm⁻¹ greater thanabout 3, a Raman sp³ full width half maximum less than about 6 cm⁻¹ anda diamond-to-graphite Raman ratio greater than about 25, comprising thesteps of:preparing said substrate by abrasion with diamond particles;placing said substrate in a CVD reactor suitable for diamond deposition;depositing diamond in said CVD reactor during a first deposition stageby providing an atmosphere consisting essentially of a mixture of about200 sccm H₂ and 10 sccm CH₄, at a pressure of about 90 Torr, providingbetween about 1,800 and 1,950 watts of microwave power at a frequency ofabout 2.45 GHz to ignite and sustain a plasma in the region of saidsubstrate, and maintaining said substrate at a temperature of betweenabout 625° C. and 675° C. for a period of time sufficient to form adiamond layer which is substantially continuous; depositing diamond insaid CVD reactor during a second deposition stage by providing anatmosphere consisting essentially of a mixture of about 200 sccm H₂, 4.6sccm CO, and 9 sccm of CH₄ at a pressure of about 90 Torr, providingbetween about 1,800 and 1,950 watts of microwave power at a frequency ofabout 2.45 GHz to ignite and sustain a plasma in the region of saidsubstrate, and maintaining said substrate at a temperature of betweenabout 625° C. and 675° C. for a period of time sufficient to form adiamond layer having a desired thickness; and removing said substratefrom said CVD reactor.
 2. The method of claim 1, further including thestep of separating said diamond layer from said substrate.